export  function canvasDataURL(path,obj,callback) {
    var img = new Image();
    img.src = path;
    var self = this
    img.onload = function () {
      var that = this;
      // 默认按比例压缩
      var w = that.width,
        h = that.height,
        scale = w / h;
      w = obj.width || w;
      h = obj.height || (w / scale);
      var quality = 0.7;  // 默认图片质量为0.7
      //生成canvas
      var canvas = document.createElement('canvas');
      var ctx = canvas.getContext('2d');
      // 创建属性节点
      var anw = document.createAttribute("width");
      anw.nodeValue = w;
      var anh = document.createAttribute("height");
      anh.nodeValue = h;
      canvas.setAttributeNode(anw);
      canvas.setAttributeNode(anh);
      ctx.drawImage(that, 0, 0, w, h);
      // 图像质量
      if (obj.quality && obj.quality <= 1 && obj.quality > 0) {
        quality = obj.quality;
      }
      // quality值越小，所绘制出的图像越模糊
      var base64 = canvas.toDataURL('image/jpeg', quality);
      // 回调函数返回base64的值
      alert(base64)
      callback(base64)
    }
  }


